1. Field of the Invention
The present invention relates to a polarity converter for a parabolic antenna of the kind used in receiving satellite broadcasts or the like.
2. Description of the Background
In order to make it easier to install a parabolic antenna for receiving electromagnetic waves transmitted by a broadcast satellite, that is, in order to allow the receiving parabolic antenna to be installed without taking the polarity of the electromagnetic waves into consideration, such electromagnetic waves are typically transmitted from the satellite with a circular polarization. Thus, it is necessary to convert the electromagnetic waves with a circular polarization into ones with a linear polarization in order to efficiently transform the electromagnetic waves into an electrical signal. For this reason, a polarity converter is required when using a parabolic antenna.
FIG. 6 shows a typical configuration of a conventional polarity converter, in which a waveguide 2 is connected to a feedhorn 1, which has a circular cross section. A dielectric substance 6 is attached across the inside of a portion 3 of the waveguide 2 that is closest to the feedhorn 1. The dielectric substance 6 is fixed at an angle at a point along the length of the waveguide 2 on a diametrical line of the waveguide portion 3, the cross section of which is circular as described above. This dielectric substance 6 is used for converting the circular polarity of the received electromagnetic wave into a linear polarity.
A portion 5 of the waveguide 2 at the stage farthest from the feedhorn 1 is designed so that it is rectangular in cross section to facilitate the transmission of the electromagnetic waves with the linear polarity. A waveguide portion 4 between the portions 3 and 5 is a transition part of the waveguide 2 at which the circular cross section is gradually transformed into a rectangular cross section. Thus, the waveguide portion 4 linking the portions 3 and 5 to each other has a cross section which is a transition between the other two.
The conventional polarity converter is designed as a three-dimensional structure for converting circular-polarity electromagnetic waves into linear-polarity electromagnetic waves. As a result, the conventional polarity converter has several problems, such as large size and high cost to manufacture.